Titanium casting process

ABSTRACT

Method and apparatus for casting a titanium alloy which involves suspending a slug of the alloy in spaced relation to the walls of a refractory crucible, inductively heating the slug while so suspended to eventually melt the same, collecting the molten alloy on a disc of titanium alloy positioned in the crucible, and then continuing the application of heat to the molten metal while supported on the disc until the molten metal melts the disc and can then be directed to a mold.

United States Patent 3,435,878 4/1969 Howardetal.

Inventor Clyde C. Clark Eneld, Ohio Appl. No. 767,168 Filed Oct. 14,1968Patented Aug. 10, 1971 Assignee TRW Inc.

Cleveland, 080

TITANIUM CASTING PROCESS 4 Chi, 3 Ih'lwhg l b.

US. Cl. 164/51, 75/10, 75/135, 75ll75.5 ht. CL 822d 27/02 Field ofSearch 164/1 19, 51, 50, 48; 75/10, 135, 175.5

References Cited UNITED STATES PATENTS 2,871,533 2/1959 Swainson 75/10X3,060,065 10/1962 Orem 75/10 R 3,529,958 9/1970 Buehler.... 164/51 UX2,091,087 8/1937 Wempe 75/10 R 3,226,223 12/1965 Bussard et a1... 75/10R 2,866,700 12/1958 Bohnet etal 75/10 R I Primary Examiner-J. SpencerOverholser Assistant Examiner-V. K. Rising Attomey1-1ill, Sherman,Meroni, Gross and Simpson ABSTRACT: Method and apparatus for casting atitanium alloy which involves suspending a slug of the alloy in spacedrelation to the walls of a refractory crucible, inductively heating theslug while so suspended to eventually melt the same, collecting themolten alloy on a disc of titanium alloy positioned in the crucible, andthen continuing the application of heat to the molten metal whilesupported on the disc until the molten metal melts the disc and can thenbe directed to a mold.

BACKGROUND OF THE INVENTION 1. Field of the Invention This applicationdescribed improvements to the subject matter which is described andclaimed in U.S. Pat. No. 3,484,840 issued Dec. 16, 1969 to Spoth andChandley and entitled Method and Apparatus for Melting and PouringTitanium."

This invention is in the field of melting titanium alloy (70 percenttitanium and above), and including commercially pure titanium (99.8percent Ti). The invention includes the step of bringing a slug of thetitanium alloy near its melting temperature while suspended out ofcontact with the walls of the crucible, and then applying additionalheat to the slug to melt the same. The molten metal is then superheatedwhile it is disposed on a disc of a titanium alloy which is preferablyof the same chemical composition as the material of the slug. continuedapplication of heat causes the molten metal to melt through the titaniumalloy disc and the molten titanium can then be introduced into a mold.

2. Description of the Prior Art Titanium alloys used in forming shapedcastings are usually skull melted in water-cooled, crucible arc meltingunits. The temperature control in this type of melting is difficultbecause the molten titanium is cooled rapidly by the water-cooledcrucible when power from the arc is terminated. A resulting variation inpouring temperature may cause many casting defects. If the temperatureis too high, dimensional and metallurgical problems result. If thepouring temperature is too low, the castings misrun and also containmetallurgical defects.

One of the most serious problems in melting and pouring titanium is theavoidance of contamination. Titanium has a strong affinity for hydrogen,nitrogen, oxygen and carbon. Excess absorbed hydrogen is removed fromtitanium alloys by expensive processing such as vacuum annealing.According to current specifications, the hydrogen content of titaniumshould be limited to about 125 and 200 parts per million. Above theselimits, hydrogen embrittles most titanium alloys, and reduces theirimpact strength and notch tensile strength, causing brittle failureunder sustained loads at low stresses.

Other contaminants cause titanium alloys to become embrittled. Thisembrittlement is usually manifested by the formation of a brittlesurface layer which must be removed by pickling, grinding or machining.

The drawbacks to commercial skull melting procedures as presentlypracticed in the casting of titanium are further described in thearticle entitled A Look at Worldwide Titanium Technology" appearing inthe Sept. 1968 issue of Metal Progress, pages 60 to 71.

SUMMARY OF THE INVENTION The present invention relates to a method andapparatus for melting and pouring titanium alloys in which a slug of thetitanium alloy to be melted is first suspended in a refractory crucibleso that it is out of contact with the walls thereof. The crucible ispreferably made of dense graphite, and has an aperture in the basethereof. The aperture is covered by means of a disc of titanium alloyhaving the same chemical analysis as the slug to be melted, and in anyevent, being devoid of any contaminating elements which would affect thephysical properties of the titanium melt. The slug of titanium alloy isinductively heated while so suspended to a temperature just below themelting point of the slug. When the entire slug is substantially at thistemperature, an additional amount of heat is provided by the inductionheating apparatus to raise the tempera ture of the slug above themelting temperature, whereupon a molten pool of titanium alloy is formedon the titanium alloy disc. The molten alloyis superheated and, in ashort period of time, it melts through the titanium alloy disc and runsout of the crucible through the aperture. The time of contact of themolten titanium alloy with the refractory crucible is thus held to aminimum.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a somewhat schematic view ofa melting and pouring assembly which can be used in accordance with thepresent invention, illustrating the manner in which the titanium alloyslug is suspended in the crucible before melting;

FIG. 2 is a fragmentary view similar to FIG. 1 but illustrating theinterior of the crucible after the slug has been melted; and

FIG. 3 is a view similar to FIG. 2 but illustrating the condition of theassembly after the molten titanium alloy has melted the titanium alloydisc.

DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention isapplicable to the melting and pouring of titanium alloys generally,where contamination is a problem. It is applicable to the melting ofcommercially pure titanium (99.2 percent Ti) as well as to alphatitanium alloys (Ti-5Al-2.5Sn) to alpha-beta titanium alloys(Ti-2Fe-2Cr-2 Mo) (Ti-8Mn) (Ti-4Al-Mn)(Ti-4Al-3Mo-1V) (Ti-5A1-l.5Fe-l.4Cr-l.2Mo) (Ti-6A1-4V) (Ti-7A1-4Mo) and to beta titanium alloys(Ti-3A1-1 3V-l lCr).

Turning now to the drawings, in FIG. 1 reference numeral 10 indicatesgenerally a melting and pouring apparatus which can be used for thepurposes of the present invention. The apparatus 10 includes a graphitecrucible 11 having a neck 12 providing a discharge opening 13. Thegraphite crucible 11 is positioned on a ceramic support plate 14 havingan aperture 15 therein accommodating the neck 12 of the crucible. Acylindrical ceramic heat shield 16 is positioned about the graphitecrucible 11 in coaxial relation. Surrounding the heat shield 16 is ahigh frequency induction coil 17 connected to a suitable source of power(not shown). The coil 17 rests on one or more positioning blocks 18standing on the ceramic support plate 14.

Overlying the discharge opening 13 is a disc 19 of titanium alloy, thedisc 19 having a reduced thickness central portion 20. A slug 21 of thetitanium alloy to be melted'is supported in spaced relation to the wallsof the crucible 11 as well as the disc 19 by a support rod 22 which issuitably fastened to the slug 21 as by means of a threaded engagement. Athermocouple 23 is positioned centrally of the slug 21 to give anindication of the temperature at that portion of the slug.

When the slug is supported as illustrated in FIG. 1 of the drawings,sufficient power is applied to the coil 17 to preheat the entiresuspended slug to a temperature just below its melting point. Themelting points of commercial titanium alloys range from about 2800" to 3135 F. for the commercially pure titanium. With induction heating, theheat buildup occurs on the outer surface adjacent to the induction coil17 and progresses inwardly, mainly by conduction. The progress of theheating is monitored by the thermocouple 23. When the slug is uniformlyheated to a temperature within say F. of its melting point, full poweris applied to the coil 17, and melting is completed in a very shorttime. The molten metal 24, as illustrated in FIG. 2 of the drawings, iscollected momentarily by the titanium alloy disc 19. This permits themelt to reach the desired superheat of 100 to 400 F. or so The superheatcan be controlled by the thickness of the reduced portion 20 of the discand its location with respect to the coil 17. The molten bath 24 rapidlymelts the disc 19 as illustrated in FIG. 3, and proceeds through thedischarge opening 13 into the gate 25 of a mold assembly for producingcastings.

It such be understood that the melting and pouring is done undernoncontaminating conditions such as under vacuum conditions or in thepresence of an inert gas suc as argon. Since such melting and pouringconditions are customary for titanium alloys, the furnace enclosureshave not been illustrated in the drawings.

It has been found through the use of the method and apparatus of thepresent invention wherein the slug is preheated without cruciblecontact, that a much shorter molten cycle results, and considerably lesscarbon contamination is encountered.

It should be evident that various modifications can be made to thedescribed embodiments without departing from the scope of the presentinvention.

1 claim as my invention:

1. The method of casting a titanium alloy which comprises suspending aslug of said alloy in spaced relation to the walls of a refractory,electrically conductive crucible, inductively heating said slug while sosuspended until the entire slug reaches a substantially uniformtemperature slightly below the melting point, thereafter increasing theamount of heat applied to raise the temperature of said slug above itsmelting point, collecting the molten alloy on a disc of titanium alloy,continuing the application of heat to the molten metal while supportedon said disc until the molten metal melts through said disc anddirecting the molten metal to a mold.

2. The method of claim 1 in which said crucible is composed of graphite.

3. The method of claim 1 in which said disc is composed of the samealloy as said slug.

4. In a method of casting a titanium alloy, the steps comprisingsuspending a slug of said alloy in spaced relation to the walls of arefractory electrically conductive crucible, inductively heating saidslug until the slug reaches a substantially uniform temperaturethroughout, which temperature is within about F. of the slug meltingpoint, and increasing the quantity of inductive heating to rapidly meltthe slug and thereby create a molten pool of the alloy in said crucible.

2. The method of claim 1 in which said crucible is composed of graphite.3. The method of claim 1 in which said disc is composed of the samealloy as said slug.
 4. In a method of casting a titanium alloy, thesteps comprising suspending a slug of said alloy in spaced relation tothe walls of a refractory electrically conductive crucible, inductivelyheating said slug until the slug reaches a substantially uniformtemperature throughout, which temperature is within about 100* F. of theslug melting point, and increasing the quantity of inductive heating torapidly melt the slug and thereby create a molten pool of the alloy insaid crucible.